hydra presses

8/9/2019 HYdra Presses

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An innovative approach to solve current

problems in advanced forming using

Proportional Hydraulic Technology with PLC(advanced hydraulic presses)

Sanjeev.R1, Prafull V.Joshi

2, Dr. S.V Prakash

3, Prof. K.R Prakash

4

1, 24

thSem., M.Tech-CIM,Dept. of Mechanical Engg., MSRIT, Bangalore.

3Asst.Prof. Dept. of Mechanical Engg., MSRIT, Bangalore

4Special Officer, VTU-BR CoC, Mysore.

Abstract: - Hydraulic presses always offered tremendous advantages to metal formers in terms of flexibility and control of ramspeed and stroke. Fast open-loop controls monitor valve activity every few milliseconds, so that state-of-the-art proportional valves

precisely control and direct hydraulic oil to the press. Proportional hydraulic technology is rapidly convincing metal formers that now

are the time to retire aging presses and bring in new models.

In this paper an attempt is made to improve the performance of the metal forming presses incorporating proportional hydraulic valves

over conventional hydraulic systems. This paper concentrates on position control and velocity control of ram. A circuit has been builtusing 4/3 proportional directional control valve and a load pressure valve to simulate ram movements

Here velocity control of the ram i.e. approaching a part quickly, and then varying speed during the forming process and again

retracting quickly, is achieved by assigning various command values (analog inputs) and ramps through on board electronics to the

valve. Positional control of the ram is through adjusting the braking distance by decelerating the ram movement. This braking

distance can be varied by assigning different ramp values (decelerating time) to the valve.The programmable logic controllers (PLCs) and interfaces available along with sophisticated proportional valves, can deliver more

capabilities. This results in greater precision during forming and greater control of the forming process. In above experimentation

velocity and position control of the ram is achieved using PLC program and sensors.

Index Terms: Hydraulic presses, Proportional Directional control valve, PLC programming, Braking distance, position control,

velocity control


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I. Introduction:

Increasing pressure on metal formers to improve

productivity while decreasing costs has placed stringent

demand on the machines they employ. Hydraulic remains

the power of choice for heavy duty presses, but olderhydraulic control systems often lack the precision to

produce highly accurate and repeatable motion. Also

more and more, hydraulic presses are incorporated into

assembly cells or automation production lines, which put

constant demand on the presses to increase productivity,

improve quality, faster cycle times and decrease

maintenance cost. [1]

Choosing the proper hydraulic elements plays a vital role

to meet the above put requirements. Heavy duty (high

tonnage) presses involve large masses to be moved such

as movable platen, dies etc. Press motions involve

movement of these large masses at various speeds

changing continuously. Conventional hydraulic systemswith ON-OFF valves are inefficient for such applications

since they create large acceleration and deceleration

forces during changeover of directions, which may cause

major system damages. Complex motion requires the

conventional system to incorporate many components(FCVs, check valves etc.) increasing system complexity,

which directly affects product quality and productivity.

Open loop control of hydraulic presses has provided a

new edge in terms of performance and more accurate

control with improved reliability. These open loopcontrols incorporate Proportional Hydraulic valves where

in they monitor valve activity every few milliseconds, so

that state of art proportional valves precisely control and

direct hydraulic oil to press. Proportional hydraulic

technology is rapidly convincing metal formers that now

are the time to retire aging presses and bring in new

models.

Proportional hydraulic ales offer various advantages over

conventional ON -OFF valves that includes thefollowing: - [2]

The technical benefits of the proportional devicescan be primarily found in the controlled transfer

during valve changeover, i.e. infinitely variable

control of command signals (i.e. speed) and the

reduction of hydraulic equipments requirements for

certain control applications. This therefore also

represents an effective contribution to reducing

material requirements in the hydraulic circuits.

Proportional valves and pumps with theirproportional solenoids provide perfect interface for

electronic control, thereby facilitating increased

flexibility in the operating cycles of production

machines as well as freely programmable controlsystems and drives.

Proportional valves permit faster, simpler, andprecise movement cycles while at the same time

improving the reversal process. As a result of

controlled spool cross-over, pressure peaks areavoided resulting in a longer service life of the

mechanical and hydraulic components.

The fact that the signals for direction and flow orhydraulic pressure are provided by electrical means

has made it possible to arrange the proportionaldevices directly on the loads, thereby greatly

improving the dynamic characteristics of the

hydraulic system.

The addition of a Programmable Logic Controller (PLC)with proportional valve technology can be used to

precisely control position, velocity and force which bring

several advantages: [7]

More flexible production results from the ability toeasily change a recipe that specifies different

positions during the press cycle, which also

significantly reduces machine downtime.

Multiple sections or axes of a large press can becoordinated or synchronized.

The press can adapt automatically to handle varyingmaterial consistencies and the effects of differing

environmental conditions such as temperature and

humidity.

Pressure spikes that damage sensors and cause leakscan be reduced or eliminated, decreasing

maintenance costs and extending press life.

In addition, more consistent, smooth motion results in

less wasted or rejected parts, and ensure consistent

quality and production from different machine operators

with varying skill levels. This results in greater precision

during forming and greater control of the forming

process, with a smoother speed/position profile that

minimizes press shock and vibration. We can ramp the

control signal being sent to the hydraulic valves and

create gradual changes in hydraulic-oil flow rate which

More reliable control makes process changes morequickly, so what might have been, using previous control

technology, a step response between speed changes

becomes a smooth response.

II. Proportional Valve Technology

The emerging open and closed-loop control technology

with its high flexibility necessitated electronically

controllable hydraulic valves. Servo-valves meet thisrequirement, but were much to complex and expensive

for most of the applications. Hydraulic valves had

therefore to be developed, which can be steplessly

adjusted by means of an electronic system, while being

simple, rugged and cost-efficient. This was the "birth" ofproportional valve technology. [4] Decisive features of

proportional valves are spools with control lands, specific

solenoids and special electronics for optimum operation

of the valves. With the help of the electronics it was now

possible to steplessly adjust the valve aperture (command

value) and the spool velocity (ramp), which allowed the

machine speed and in particular acceleration to be exactly

controlled. Now, it was possible to adjust the machine

movement continuously and proportionally to electrical

signals. This resulted in the designation "proportional

valve. This technology provided higher speeds and


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hence higher productivity. Another aspect was that the

motion sequences could be programmed with an

electronic control. With today's proportional valves, the

valve electronics can be mounted directly onto the valve.

Fig 1 Signal flow in proportional hydraulics

Fig. 2 Modern proportional directional valve with integrated

On-Board Electronics (OBE)

III. Design and mode of operation of a proportional

solenoid

Depending on the design of the valve, either one or two

proportional solenoids are used for the actuation of an

electrically variable proportional valve.

A. Solenoid design

The proportional solenoid (fig. ) is derived from the

switching solenoid, as used in electro-hydraulics for the

actuation of directional control valves. The electrical

current passes through the coil of the electro-solenoid and

creates a magnetic field. The magnetic field develops a

force directed towards the right on to the rotatable

armature. This force can be used to actuate a valve.

Similar to the switching solenoid, the armature, barrel

magnet and housing of the proportional solenoid are

made of easily magnetisable, soft magnetic material.

Compared with the switching solenoid, the proportional

solenoid has a differently formed control cone, which

consists of non-magnetisable material and influences the

pattern of the magnetic field lines.

B. .Mode of operation of a proportional solenoid

With the correct design of soft magnetic parts and control

cone, the following approximate characteristics are

obtained. The force increases in proportion to the current,

i.e. a doubling of the current results in twice the force on

the armature. The force does not depend on the positionof the armature within the operational zone of the

proportional solenoid.

.


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IV. Characterization of Proportional directional valve 4WREE6E08/2X

Fig 3. Proportional directional valve 4WREE6E08/2X

Fig. 4 Symbol of a proportional valve

A. Position control and velocity control withproportional hydraulic valves

Velocity control [2]

Velocity of the cylinder (ram) is controlled from the

proportional directional valve is controlled by analog

command value module. On the command value

module, the velocities are adjusted by means of

command value potentiometers. Acceleration anddeceleration rates are adjusted by means of ramp

potentiometers.

Potentiometers "w1-4are used for adjusting the

command values and hence also the cylinder velocities.

The set values are activated by means of command

value call-ups. Command values "w1and "w2are

positive, whereas "w3and "w4are negative. In this

way, a cylinder can, for example, extend with "w1and

"w2and retract with "w3and "w4. A ramp is

assigned to each command value "w1 "t1, "w2

"t2, etc. This means that when command value

"w1is active, only ramp "t1" is also activated. Ramp"t5is active, when no command value is active.


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force via the blank holder onto the blank & the draw

die. The die & the ejector are located in the lower die

on the press bed. During forming, the blank holder

brings the sheet metal into contact against the die; the

punch descends from above into the die & shapes the

part while the sheet metal can flow without any

wrinkling out of the blank holding area. In this Case,

the drawing process is carried out with a fixed blank

holder & moving punch. In double action drawing

operations, the drawing slide can only apply a pressing

force. [6]

Fig. 6 Deep Drawing Process

A. Conventional Hydraulic circuit for deep drawing

process control

A hydraulic circuit is built with ON-OFF valves which

are as shown below. This circuit incorporates two

cylinders for typical deep drawing operation. One

cylinder acts as a blank holder and other acts as a draw

punch. The circuit incorporates solenoid operated 4/2

spring return ON-OFF valves.

The conventional hydraulic circuit is as shown below.

Fig. 7 & 8

Electrical circuit for above hydraulic circuit is shown in

Fig. 9


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Fig. 7 Conventional hydraulic circuit for rapid speed- creep speed control

Fig. 8 Hydraulic circuit for draw punch


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Fig. 9 Electrical Circuit for conventional press operation

Operation is as follows-

To reduce the idle time the blank holder cylinderis moved with rapid speed and then with creep

speed to hold the blank. The sensing action at

changeover during rapid speed to creep speed is

achieved with the help of Inductive proximity

sensors.

Once the blank is hold firmly the draw punchcylinder starts operating.

Once the drawing action is complete the drawpunch is retracted and then the blank holder

cylinder so that part can be ejected.

The speed control of cylinders is achieved with the help

of flow control valves. One flow control valve each is

required for both the cylinders.

The circuit involves two major operations-

Rapid speed- creep speed of blank holdercylinder

Advancing of draw punchHowever the deep drawing press with conventional

valves has many limitations. It incorporates three 4/2

solenoid operated spring return DCV and two flow

control valves to perform the operation. The speed

control is very cumbersome as it is with manual flow

control valves which are not accurate. Position control is

also difficult and is inflexible.

B. Hydraulic circuit with proportional valveHence to overcome all these limitations proportional

valve is incorporated through which both velocity control

and position control is achieved with a single valve,

eliminating number of valves and reducing system

complexity.

Hydraulic circuit dig with proportional valve is shown in

Fig.10 & 11

Electrical circuit dig with proportional valve is shown in

Fig.12 & 13

The operation sequence with proportional valve is as

below

The blank holder cylinder is extended rapidlywith command value w1=5V

With activation of command value w2=2V thespeed is reduced to creep speed

Then the draw punch cylinder is extended It is then retracted with sensor actuation Finally blank holder cylinder is retracted rapidly

with command value w3


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Fig.10 Hydraulic circuit with proportional valve for rapid speed creep speed control


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Fig 11 Hydraulic circuit for draw punch

Fig. 12 Electrical Circuit 1 for proportional valve


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Fig. 13 Electrical Circuit 2 for proportional valve (command value module)

VI. Proportional hydrualic system with PLC

PLC programming techniques is used to automate the

above proportional hydraulic system for deep drawing

press applications.IndraWorks/ IndraLogic programming

software with Rexroth Bosch L20 controller is used.

Ladder logic programming method is used to write the

logic.Complexity of the electrical circuitry is reduced by

PLC and allows flexibility in changing both analog and

digital control of the valves.

Fig: 14 Equivalent PLC logic diagram (Program) for above electrical circuit for proportional valve


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VII. Conclusion:

Forming presses for advanced applications will have

complex hydraulic circuits to achieve determined

characteristics, such as ram movements with creep

speeds. Use of proportional valve avoids multiple valve

incorporation. With open loop systems, velocity controlwill be accurate, yielding greater part accuracy. Position

control with proportional valves reduces large

acceleration and deceleration forces generated during

faster ram movements reducing the risk of system

damages. This enhances product quality and smoother

action of the press increasing reliability.

In recent days proportional technology along with PLC is

being used for applications which involve heavy masses

and require better control, such as deep drawing of fire

extinguishers that are 1 kg and larger, sheet steel with

thicknesses between 1.4 and 2.5 mm.

In this paper an attempt is made to use proportional valve

technology in hydraulic presses for advanced forming

process applications and automated with PLCprogramming which achieves flexible and advanced

forming options.

VIII. References

[1] Bill Savela, Pe, Smarter Motion controlspromise better Hydraulic Press Performance,

Metal forming / May 2008.

[2] R.Ewald, J. Hutter, The Hydraulic TrainerVolume 2 Proportional and Servo Valve

Technology, Rexroth Bosch Pg. 11-50, 77-86.[3] Jansuz Pluta, Hydraulic Press with LS System

for Modeling of Plastic Working Operations

[4] Learning System for Automation andCommunications Proportional hydraulics

Textbook, Festo Diadctic

[5] D. Kretz, M.Reik The Hydraulic TrainerVolume 1 Basic Hydraulis

[6] By F San Martin, F Negroni, TheHydromechanical Deep Drawing Process.

[7] Brad F. Kuvin, Hydraulic Presses -SmoothOperators Metal forming / March 2002

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